In a large glass bowl, mix together beef, soy sauce, cornstarch and 1 tablespoon of sesame oil until well combined. Set aside.

Heat 1 tablespoon of oil in a large non-stick skillet over high heat. Add beef into the skillet and cook until they're done. Transfer beef to a bowl and set aside.

Add another 1 tablespoon oil into the skillet, toss onion and garlic, cook until onion turns translucent and fragrant. Add bell pepper strips and frozen peas, cook until the vegetables are done but still slightly firm. Add cooked soba noodles and beef into the skillet. Season with salt and pepper, stir until well mixed. Remove skillet from the heat. Divide noodles into plates and serve hot.